This invention involves a process for containment of industrial wastes, particularly nuclear wastes, in apatite ceramics.
Apatite ceramics are valuable materials for use as matrices for the containment of industrial wastes, and particularly nuclear wastes and especially those with long half-lives such as fission products or certain actinides.
In irradiated nuclear fuel treatment plants at the end of the processing there are several actinide elements with long half-lives and some lanthanides which must be contained for long-term storage in highly resistant matrices.
The materials used for the matrices must have very high chemical stability, stability to radiation, and stability to temperature, to isolate the radioactive elements from the environment and keep them in this isolated state for very long periods, due to their long period of radioactivity.
The matrix currently used for containment is glass, but recent studies have indicated that apatite ceramics have properties which are particularly suitable for long-term storage and could be used instead of glass as confinement matrices.
The apatites are compounds with the general formula:
Me10(XO4)6Y2xe2x80x83xe2x80x83(I)
in which Me is one or several metals, X represents P, V and/or Si, and Y represents one or several anions such as OH, Cl and F. Among these apatites, phosphocalcic hydroxy apatite:
Ca10(PO4)6(OH)2xe2x80x83xe2x80x83(II)
is the best known compound.
The apatites of formula (I) can have various substitutions, for the cationic sites (Me) as well as for the anionic sites (XO4 and/or Y2).
The charge equilibrium, needed because of the introduction of elements which may be monovalent, divalent, trivalent or tetravalent, is established by various associated substitutions.
For example, divalent calcium can be replaced by a rare earth which is a trivalent element. This replacement can occur in several ways:
by the coupled exchange (Ca2+, OHxe2x88x92)⇄(Ln3+, O2xe2x88x92)
by the coupled exchange (Ca2+, PO43xe2x88x92)⇄(Ln3+, SiO44xe2x88x92) 
by the coupled exchange (2Ca2+)⇄(Ln3+, Na+)
This is just an example of the multiple substitutions which are possible.
As described by BROS R., CARPENA J., SERE V., BELTRITTI A, Radiochimica Acta, 74, 1996, pages 277-282 [1], the study of the OKLO natural reactor showed that apatites containing radioelements in their structures (actinides and/or fission products) are particularly stable, thermally and chemically, even in highly irradiating media.
These apatites are resistant in radioactive waste storage conditions to more than 1,000xc2x0 C. They are chemically resistant in hydrogeological storage conditions, i.e. with a water pH which is neutral or basic. They can also withstand highly irradiating media because the radiation damage which they suffer is unstable at temperatures greater than 60xc2x0 C. Phosphocalcic apatite, for example, can restructure itself as of 60xc2x0 C.
The advantage of apatites for geological storage of industrial wastes, and particularly nuclear wastes with low, moderate or high activity, is evident, because they allow for strong chemical bonding in a matrix which is particularly stable with a continuity in geological media which has been demonstrated by studies of materials several million years old.
Massive polycrystalline shapes of these apatites are needed for containment of industrial wastes, and particularly radioactive wastes.
Until now, the preparation of massive apatite pieces containing wastes was done from powdery apatites subjected to sintering at high temperatures, i.e. greater than 1,000xc2x0 C., possibly under high pressure.
The document FR-A-2 712 726 [2] describes a process for containment of actinides and/or lanthanides in apatite, involving the preparation of a mixture of powders including at least one phosphate chosen from among calcium, lanthanide and actinide phosphate, calcium fluoride, calcium carbonate, a silicon compound and possibly one or several lanthanide or actinide oxides, thermally treating the mixture to break down the calcium carbonate and calcinating the thermally-treated mixture at a high temperature (900 to 1,500xc2x0 C.), possibly repeating the last calcination step several times after one or several intermediate grinding steps.
The document Inorganic Materials, volume 9, no. 4, 1973, pages 652-654 [3] also describes a process for manufacturing fluoroapatite silicates containing lanthanides by thermal treatment at a high temperature (1,200 to 1,350xc2x0 C.).
Other processes for manufacturing of an apatite ceramic-based waste containment matrix involve the prior preparation of apatite power, its grading and sintering according to various processes such as natural sintering, pressure-assisted sintering, and sintering after using slip.
These techniques yield mass pieces with good mechanical properties, but they require high-temperature thermal treatments involving:
high costs of energy for preparation of the matrix
partial transformation of hydroxyapatite to oxyapatite, and
difficulties in enclosing species which are volatile at the temperature of the thermal treatment in the piece of apatite.
This invention precisely involves a process for containment of industrial wastes in apatite ceramics which produces pieces with good mechanical properties but without the need for thermal treatment at high temperatures.
According to the invention, the process for containment of industrial waste in a apatite ceramic matrix involves the following steps:
a) preparing a homogeneous mixture of powders, comprising:
i) at least two calcium phosphates selected from the group consisting of Ca(H2PO4)2, Ca(H2PO4)2.H2O, Ca(HPO4), Ca(HPO4).2H2O, amorphous xcex1-Ca3(PO4)2, apatitic xcex1-Ca3(PO4)2, amorphous xcex2-Ca3(PO4)2, apatitic xcex2-Ca3(PO4)2, Ca4(PO4)2O; and optionally
ii) at least one compound selected from the group consisting of alkaline metal salts, alkaline metal phosphates, alkaline metal silicates, alkaline metal carbonates, alkaline metal halides, alkaline metal oxides, alkaline metal hydroxides, alkaline-earth metal salts, alkaline-earth metal phosphates, alkaline-earth metal silicates, alkaline-earth metal carbonates, alkaline-earth metal halides, alkaline-earth metal oxides, alkaline-earth metal hydroxides, and oxides of silicon,
wherein
the mixture is able to form a stoichiometric hydroxyapatite of formula (II)
Ca10(PO4)6(OH)2xe2x80x83xe2x80x83(II)
wherein
calcium is partly replaced by at least one member selected from the group consisting of alkaline metals and alkaline-earth metals;
phosphate anions are partly replaced by silicate anions; and
hydroxide anions are party replaced by halide anions;
b) putting the industrial wastes into the mixture;
c) compacting the mixture of powders containing the aforesaid waste at room temperature, under a pressure of 100 to 500 MPa, to yield a compacted piece; and
d) subjecting the compacted piece to hydrothermal treatment in a sealed chamber containing an aqueous medium, at a temperature of 100 to 500xc2x0 C., for a period of at least 8 hours.
According to a first embodiment of the invention, particularly intended for containment of industrial wastes including at least one element chosen from the metals and halogens, steps a) and b) are done simultaneously by mixing the waste, during preparation of the mixture of powders, in the form of powders of oxides, hydroxides or salts of the metal(s) and/or alkaline or alkaline earth metal halide powder(s) so as to obtain a mixture corresponding to a hydroxyapatite as defined above, substituted by the metal(s) and/or halogens to be contained.
The metals could in particular be radioactive metals such as radioactive cesium, for example Cesium-135 and Cesium-137, Strontium-90, Technetium-99, the lanthanides, particularly Samarium-151, and the actinides. The halogens could be Chlorine-36 in particular.
According to a second embodiment of the invention more particularly intended for containment of wastes in the form of powders, granulates, massive pieces of variable size or organic wastes, these are put as is into the mixture of powders prepared in step a) so that they are surrounded by the mixture of powders.
Waste of this type can be made for example of powders, granulates, or small massive pieces of apatite or ceramics containing radioactive elements, pre-treated wastes, contaminated technological wastes such as pieces of metal, metallic drums, glass, etc. and of organic materials such as asphalt which contain radioactive elements or other elements.
This second embodiment of the invention can be combined with the first when simultaneously enclosing waste which can enter the chemical structure of the apatite and the other wastes.
The invention process thus allows for preparation of the apatite ceramic matrix at low temperatures, using a hydrothermal reaction between various phosphatised compounds and possibly other compounds present in the mixture, which were first compacted.
In step a) of this process, a mixture of powders which can yield a hydroxyapatite with the following formula is prepared:
Ca10(PO4)6(OH)2
in which the anions and/or cations can be substituted by other cations and anions, and in particular by the element(s) of the waste to be contained.
This hydroxyapatite can in particular be a silicated apatite such as those described in FR-A-2 712 726 [2], which may or may not contain lanthanides and/or actinides in its structure.
The mixture can be prepared by grinding the ingredients to a size grading of less than 100 xcexcm. Some components, such as calcium phosphates, can be in the form of a single powder obtained by co-grinding.
According to the invention, the mixture includes at least two phosphate compounds, in particular a basic compound (tetracalcium phosphate) and one or several acidic compounds (dicalcium or monocalcium phosphate). Phosphatised compounds, oxides, hydroxides and salts of alkaline metal or alkaline-earth metals or metals forming the waste to be contained can also be added to provoke various substitutions in the hydroxyapatite.
The salts used can be in particular phosphates, silicates, nitrates, halides or carbonates.
The mixture is then subjected to compacting step c) after introduction of the waste for containment, if it is not part of the mixture.
The compacting is done at room temperature, for example at a temperature of 15 to 30xc2x0 C., under a pressure of 100 to 500 MPa, preferably 200 MPa, for example by means of a hydraulic press, after putting the mixture into a mould.
In the next step d), the compacted piece is subjected to a hydrothermal treatment in a sealed chamber in the presence of an aqueous medium brought to a temperature of 100 to 500xc2x0 C., under a pressure which corresponds to the pressure of water vapour at the chosen temperature.
This treatment yields a ceramic form by hydrothermal reaction between the ingredients in the compacted mixture. Pieces with exceptional hardness can thus be obtained because acicular crystals of apatite which condition the cohesion of this material have developed within the massive material.
The hydrothermal treatment can be done in two ways.
According to a first embodiment of the treatment, the compacted piece is totally immersed in the aqueous medium so that it is in contact with the water in the liquid state.
According to a second embodiment of this hydrothermal treatment, preferably used for compacted pieces including compounds which are soluble in aqueous media, the compacted piece is arranged above the liquid medium so that it is only in contact with the water vapour produced within the sealed chamber under the effect of the treatment temperature.
The hydrothermal treatment temperature is between 100 and 500xc2x0 C., and the duration of this hydrothermal treatment depends in particular on the temperature used, the duration being longer when the temperature is lower. The duration is generally at least 8 hours and can be from 12 to 60 hours.
The hydrothermal treatment temperature is preferably 150 to 250xc2x0 C. for a period of about 48 hours.
The aqueous medium used is usually demineralised water, but an aqueous solution containing appropriate additives could also be used.
According to a variant of the invention process embodiment, there is an additional step e) of sintering of the compacted piece which was subjected to hydrothermal treatment. This sintering is done at a temperature of at least 1,000xc2x0 C., for example between 1,000 and 1,300xc2x0 C.
Highly compacted materials with excellent mechanical properties can thus be obtained for safe containment of wastes for long-term storage.
The invention process is particularly advantageous because it can yield various compositions of apatite ceramic matrices by choosing the compounds used in step a).
When an apatite ceramic matrix of the phosphocalcic hydroxyapatite type is to be prepared, a mixture of various calcium phosphate compounds is used such as Ca(H2PO4)2, Ca(H2PO4)2.H2O, amorphous xcex1Ca3(PO4)2, apatitic xcex1-Ca3(PO4)2, amorphous xcex2-Ca3(PO4)2, apatitic xcex2-Ca3(PO4)2, and Ca4(PO4)2O; The calcium phosphate compounds may be in proportions such that the fin composition is that of a hydroxyapatite of the formula:
Ca10(PO4)6(OH)2xe2x80x83xe2x80x83(II)
wherein
calcium is partly replaced by at least one member selected from the group consisting of alkaline metals and alkaline-earth metals;
phosphate anions are partly replaced by silicate anions; and
hydroxide anions are partly replaced by halide anions.
Starting with a mixture of powders including compounds other than these calcium phosphates, apatites can be made with substitution:
of the cation Me, with strontium for example,
of the PO4 group, with silicate groups for example,
on the OH anion, with fluoride or chloride ions for example.
Some of the compounds used can have elements from wastes such as radioactive elements such that, at the end of the operation, an apatite ceramic matrix is obtained which encloses radioactive elements in its structure, thus allowing for their containment for long-term storage.
The invention process can also use the two techniques for incorporation of wastes by including part of them in the chemical structure of the apatite matrix and another part of them in the mixture of powders which undergoes compaction.
Other characteristics and advantages of the invention will be clearer with a reading of the following examples which are purely illustrative and in no way limiting.